In a network-based communications environment, a communications standard, such as, for example, synchronous optical network (SONET), may be employed for supplying connectivity between a plurality of nodes in the network. SONET is a well-known standard for optical telecommunications transport promulgated by the Exchange Carriers Standards Association (ECSA) for the American National Standards Institute (ANSI). The SONET standard is set forth in the document ANSI T1.105-1988, entitled “American National Standard for Telecommunications—Digital Hierarchy Optical Interface Rates and Formats Specification” (September 1988), which is incorporated by reference herein. Additional supplements to the SONET standard, including ANSI T1.105.01-2000, entitled “Synchronous Optical Network (SONET)—Automatic Protection Switching” (2000), are also incorporated by reference herein.
A SONET is commonly implemented as a ring architecture, such as, for example, a Unidirectional Path Switched Ring (UPSR). An illustrative UPSR 100 is shown in FIG. 1. A ring, unlike a linear add/drop chain, is generally defined as a set of nodes 102 interconnected by optical fiber links 104, 106 to form a closed loop. A node 102 in the SONET context typically comprises an add/drop multiplexer (ADM) configured to allow signals to be added into or dropped from a transported data frame. In the UPSR, data traffic is always routed in one direction (e.g., counterclockwise). The UPSR provides redundant bandwidth to protect services against node failures, or other failure conditions, for improved transport survivability (e.g., self-healing) via SONET path selection.
The basic building block in a SONET is a synchronous transport signal level-1 (STS-1e) frame. The STS-1 frame is transported at a 51.840 Megabits per second (Mbps) serial transmission rate using an optical carrier level-1 (OC-1) optical signal. The basic frame rate in a SONET is 8,000 frames per second, but the bit rate will depend on the frame format used. Higher-rate signals are formed by combining multiple STS-1 frames, typically by interleaving a byte from each STS-1 frame. For example, to form an STS-48 frame format, 48 STS-1 frames are multiplexed together. The basic STS-48 frame rate remains at 8,000 frames per second, but the data capacity will be 48 times greater than an STS-1 frame. The STS-48 may then be converted to an optical carrier signal (OC-48) for transport, or further multiplexed to form higher capacity channels.
The STS-1 frame structure is organized as nine rows by ninety columns of bytes, for a total frame capacity of 810 bytes. A transport overhead (TOH) portion occupies the first three columns of the STS-1 frame, and the remaining 87 columns form a synchronous payload envelope (SPE). The TOH portion of the STS-1 frame dedicates the first three rows for section overhead (SOH) and the remaining six rows for line overhead (LOH). SOH bytes are used primarily for framing, section error monitoring, and section level equipment communications. The LOH bytes are used to provide information relating to line protection and maintenance. LOH bytes are typically created and used by line terminating equipment (LTE), such as, for example, ADMs. The SPE contains one column dedicated to path overhead (POH), leaving the remaining 86 columns for payload data (49.536 Mbps). Four different size payloads called virtual tributaries (VT) fit into the SPE of the STS-1 frame. These are: VT1.5, which is 1.728 Mbps; VT2, which is 2.304 Mbps; VT3, which is 3.456 Mbps; and VT6, which is 6.912 Mbps. Each VT requires a 500 microsecond (μs) structure (e.g., four STS-1 frames) for transmission.
Each layer in the SONET signal provides alarm and error monitoring capabilities between various terminating points in the network. In a VT level of the UPSR, one OC-48 signal may contain a maximum of 1,344 channels. UPSR status information is conventionally generated by internal POH monitors. If a status of all channels associated with a given signal (e.g., OC-48) is required, all channels must be monitored, which is difficult and costly to implement in a single integrated circuit (IC) device. In a device capable of providing the status of all channels using traditional means, a significant portion of the hardware necessary for monitoring all of the channels may be undesirably wasted, since in many applications not all channels need to be monitored. Likewise, in a device capable of monitoring only a subset of the total number of channels, the device may not be fully functional in certain applications in which the status of all channels is required.
Accordingly, there exists a need for improved techniques for providing a status of all channels associated with a given signal in a network-based data transport system that does not suffer from one or more of the problems exhibited by conventional methodologies.